Examples: Mixture Modeling With Longitudinal Data
When TYPE=MIXTURE is specified, either user-specified or automatic
starting values are used to create randomly perturbed sets of starting
values for all parameters in the model except variances and covariances.
In this example, the random perturbations are based on automatic
starting values. Maximum likelihood optimization is done in two stages.
In the initial stage, 20 random sets of starting values are generated. An
optimization is carried out for 10 iterations using each of the 20 random
sets of starting values. The ending values from the 4 optimizations with
the highest loglikelihoods are used as the starting values in the final
stage optimizations which is carried out using the default optimization
settings for TYPE=MIXTURE. A more thorough investigation of
multiple solutions can be carried out using the STARTS and
STITERATIONS options of the ANALYSIS command. In this example,
40 initial stage random sets of starting values are used and 8 final stage
optimizations are carried out.
MODEL:
%OVERALL%
i s | y1@0 y2@1 y3@2 y4@3;
i s ON x;
c ON x;
The MODEL command is used to describe the model to be estimated.
For mixture models, there is an overall model designated by the label
%OVERALL%. The overall model describes the part of the model that
is in common for all latent classes. The | symbol is used to name and
define the intercept and slope growth factors in a growth model. The
names i and s on the left-hand side of the | symbol are the names of the
intercept and slope growth factors, respectively. The statement on the
right-hand side of the | symbol specifies the outcome and the time scores
for the growth model. The time scores for the slope growth factor are
fixed at 0, 1, 2, and 3 to define a linear growth model with equidistant
time points. The zero time score for the slope growth factor at time
point one defines the intercept growth factor as an initial status factor.
The coefficients of the intercept growth factor are fixed at one as part of
the growth model parameterization. The residual variances of the
outcome variables are estimated and allowed to be different across time
and the residuals are not correlated as the default.
In the parameterization of the growth model shown here, the intercepts
of the outcome variable at the four time points are fixed at zero as the
default. The intercepts and residual variances of the growth factors are